Signals Intelligence (SIGINT) may be defined as reception of a signals (e.g., learning things) about those who may emit a Radio Frequency (RF) signal. SIGINT may be further broken down into Electronic Intelligence (FLINT) comprising identification and classification of enemy radar systems (e.g., radar sniffing); and Communications Intelligence (COMINT) comprising gathering information relative to the signals used for general communications purposes. Further, Intelligence, Surveillance and Reconnaissance (ISR) may be defined as activity focused on synchronization and operational deployment of assets capable of SIGINT and further dissemination of the received and processed information.
A wide variety of systems may be usable for SIGINT operations of acquiring and processing RF and DF information. Traditional ISR systems may range in size from hand-held devices to orbiting satellites. Some systems collect basic information for a wide range of analytical products while others are designed to acquire data for specific weapons systems. SIGINT systems may be global, theater based or tactical in nature and coverage.
Precision DF (PDF) Systems are traditionally based on multiple antenna elements strategically positioned relative to one another for DF algorithms to process the direction of arrival (DOA) of the intended signal. Phase Interferometer Direction Finding (PIDF), Amplitude Monopulse Direction Finding (AMPDF), Pseudo Doppler (PDDF) may be some examples of these traditional multiple antenna systems. A typical DF systems may use at least 4 antenna elements (spaced every 90° about 360° for full azimuthal coverage), plus an additional omnidirectional antenna element for general target sensing and as a communications link.
One traditional goal of COMINT may include hardware which is not easily visually detectable. A Low Observable (LO) antenna may aid operators in a successful COMINT operation. Antenna size/volume is a direct function of wavelength where antenna size is inversely proportional to frequency. For example, a classic λ/4 whip antenna measures 4.9 ft. tall @ 50 MHz which falls outside of the LO definition. In addition to physical size, electrically small antennas may function in a COMINT role however these electrically small antennas suffer from an intrinsically narrow band of coverage. For traditional FLINT (2—18+GHz) and COMINT (HF—5 GHz) the desired hardware needs to be fundamentally broadband.
Traditional DF algorithms may require sophisticated data to accurately determine a precise DF solution. For example, Precision Interferometric DF (PIDF) algorithms may require a low far field radiated phase wobble on the wave (WoW) as well as low amplitude WoW in azimuth. In addition, these algorithms require a 3 dB beam width in elevation and minimum mutual coupling between DF array elements. Also, for maximum accuracy, PIDF systems require a precision phase center that is highly repeatable between the DF radiation elements.
Traditional Interferometer DF solutions require an undesirable linear array of antennas to perform accurate DF measurements. Embedded circuitry measures phase difference of received signal between multiple antenna pairs and algorithms calculate a DF angle from phase measurements. These DF solutions are then inserted into a Pulse Descriptor Word (PDW) for downstream processing.
Amplitude Mono-Pulse DF (AMPDF) requires high element to element uniformity in: 1) 3 dB beam width, 2) peak of beam direction, 3) azimuth pattern symmetry, and 4) element gain. Most importantly though, AMPDF requires low antenna element-to-element mutual coupling to maintain accuracy.
Traditional DF array with multi-arm planar spirals have proven successful in accurate DF while being relatively small in size. However, this planar spiral architecture suffers from a loss of approximately 3 dB of gain due to half of the energy being absorbed by back pattern absorbent material and greater size requirements due to required placement of the absorbent material distant from the spiral.
Traditional multi-arm planar spiral antenna elements are too sizable in circumference and height for a given frequency band to permit successful covert SIGINT goals herein. Spiral antenna elements are unattractive below frequencies of 2 GHz for DF applications and maintain poor radiation efficiency due to absorber loading within the element cavity. Further, spiral antenna elements are expensive to produce due to the required high touch labor in manufacturing calibration during testing.
Additional traditional COMINT DF systems may employ air-loaded Transverse Electromagnetic (TEM) horn antennas. However, TEM horns suffer from poor radiation performance which severely degrades radiation efficiency (i.e., gain) in the lower frequency bands below approximately 2 GHz.
Successful covert SIGINT intelligence gathering and long-term ISR operations may require DF arrays to maintain High Direction of Arrival (DOA) precision while being Low Observable (e.g., unobtrusive, physically small, and highly portable). Additional characteristics including physically small size, low Radar Cross Section (RCS), Ultra Wide Band (UWB), and high electrical performance may be required to ensure future successful SIGINT operations. Consequently, a need remains for an alternative to large form factor antenna arrays which require increased size and undesirable visibility.